In recent years, there has been an increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this interest has been the superiority of ceramics with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities, when compared with metals.
Current efforts at producing higher strength, more reliable, and tougher ceramic articles are largely focused upon (1) the development of improved processing methods for monolithic ceramics and (2) the development of new material compositions, notably ceramic matrix composites. A composite structure is one which comprises a heterogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite structure typically comprises a ceramic matrix which incorporates one or more diverse kinds of filler materials such as particulates, fibers, rods, and the like.
There are several known limitations or difficulties in substituting ceramics for metals, such as scaling versatility, capability to produce complex shapes, satisfying the properties required for the end use application, and costs. Several copending patent applications assigned to the same owner as this application (hereinafter referred to as Commonly Owned Patent Applications), overcome these limitations or difficulties and provide novel methods for reliably producing ceramic materials, including composites. The method is disclosed generically in Commonly Owned U.S. Pat. No. 4,713,360 which issued on Dec. 15, 1987 and was based on U.S. application Ser. No. 818,943, filed Jan. 15, 1986, which is a continuation-in-part of Ser. No. 776,964, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 705,787, filed Feb. 26, 1985, which is a continuation-in-part of U.S. application Ser. No. 591,392, filed Mar. 16, 1984, all in the names of Marc S. Newkirk et al and entitled "Novel Ceramic Materials and Methods of Making the Same". These applications disclose a method of producing self-supporting ceramic bodies grown as the oxidation reaction product from a parent metal precursor. Molten metal is reacted with a vapor-phase oxidant to form an oxidation reaction product, and the metal migrates through the oxidation product toward the oxidant thereby continuously developing a ceramic polycrystalline body which can be produced having an interconnected metallic component. The process may be enhanced by the use of an alloyed dopant, such as is used in the case of oxidizing aluminum doped with magnesium and silicon for oxidation reaction in air to form alpha-alumina ceramic structures. This method was improved upon by the application of dopant materials to the surface of the precursor metal, as described in Commonly Owned U.S. patent application Ser. No. 822,999, filed Jan. 27, 1986, which is a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 747,788, filed June 25, 1985, which is a continuation-in-part of Ser. No. 632,636, filed July 20, 1984, all in the name of Marc S. Newkirk et al and entitled "Methods of Making Self-Supporting Ceramic Materials".
This oxidation phenomenon was utilized in producing ceramic composite bodies as described in Commonly Owned copending U.S. patent application Ser. No. 819,397, filed Jan. 17, 1986, which is a continuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, both in the name of Marc S. Newkirk et al and entitled "Composite Ceramic Articles and Methods of Making Same". These applications disclose novel methods for producing a self-supporting ceramic composite by growing an oxidation reaction product from a metal precursor into a permeable mass of filler, thereby infiltrating the filler with a ceramic matrix. The resulting composite, however, has no defined or predetermined geometry, shape, or configuration.
A method for producing ceramic composite bodies having a predetermined geometry or shape is disclosed in the Commonly Owned and copending U.S. patent application Ser. No. 861,025, filed May 8, 1986. In accordance with the method in this U.S. patent application, the developing oxidation reaction product infiltrates a permeable preform in the direction towards a defined surface boundary. It was discovered that high fidelity is more readily achieved by providing the preform with a barrier means, as disclosed in Commonly Owned and copending U.S. patent application Ser. No. 861,024, filed May 8, 1986. This method produces shaped self-supporting ceramic bodies, including shaped ceramic composites, by growing the oxidation reaction product of a metal precursor to a barrier means spaced from the metal for establishing a boundary or surface. Ceramic composites having a cavity with an interior geometry inversely replicating the shape of a positive mold or pattern is disclosed in Commonly Owned and copending U.S. patent application Ser. No. 823,542, filed Jan. 27, 1986, and in U.S. patent application Ser. No. 896,157, filed Aug. 13, 1986.
The aforementioned Commonly Owned Patent Applications disclose methods for producing ceramic articles which overcome some of the traditional limitations or difficulties in producing ceramic articles as substitutes for metals in end-use applications.
Common to each of these Commonly Owned Patent Applications is the disclosure of embodiments of a ceramic body comprising an oxidation reaction product interconnected in one or more dimensions (usually in three dimensions) and one or more metallic constituents or components. The volume of metal, which typically includes non-oxidized constituents of the parent metal and/or metal reduced from an oxidant or filler, depends on such factors as the temperature at which the oxidation reaction product is formed, the length of time at which the oxidation reaction is allowed to proceed, the composition of the parent metal, the presence of dopant materials, the presence of reduced constituents of any oxidant or filler materials, etc. Although some of the metallic components may be isolated or enclosed, it is frequently the case that a substantial volume percent of metal will be interconnected and accessible from an external surface of the ceramic body. It has been observed for these ceramic bodies that this interconnected metal-containing component or constituent can range from about 1 to about 40 percent by volume, and sometimes higher. Such a metallic component can impart certain favorable properties to, or improve the performance of, the ceramic articles in many product applications. For example, the presence of metal in the ceramic structure may have a substantial benefit with respect to imparting fracture toughness, thermal conductivity, resilience or electrical conductivity to the ceramic body.
U.S. application Ser. No. 896,481 discloses a further modification of the processes disclosed in the Commonly Owned Patent Applications, for modifying the metallic constituents present in the aforementioned ceramic and ceramic composite bodies. In a post-treatment process, the unoxidized parent metal which is comprised within the body is displaced with one or more foreign metals, chosen so as to effect desired improvements in the properties of the end product. In a preferred embodiment, displacement is effected by immersing the body in a molten pool of the foreign metal where this foreign metal has a higher melting point than the displaced parent metal.
The entire disclosures of all of the foregoing Commonly Owned Patent Applications are expressly incorporated herein by reference.
The present invention discloses a method for producing self-supporting polycrystalline ceramic structures whereby molten parent metal reacts with a vapor-phase oxidant to form an oxidation reaction product which grows progressively so as to form a polycrystalline ceramic structure, optionally embedding a permeable mass of filler. The structure so produced is coated with one or more materials which effect desired changes to the surface properties of the final product.